A viewing system for an aircraft includes at least one inertial unit, sensors measuring altitude and speed, electronic calculation means and one or more viewing devices incorporated within the cockpit of the aircraft and displaying a symbolic representation of the main flight and navigation parameters. Generally, the synthetic image is displayed on the viewing screens that are located on the front of the instrument panel of the aircraft or in “head-up” viewing devices.
By way of example, FIG. 1 shows, in a stylized manner and with the inherent limitations of patent figures, an image of this type. The symbolic representation is represented by black lines. This primarily includes:                “ADI” (attitude director indicator) parameters, which indicate the attitude of the aircraft, i.e. the angular position thereof in terms of roll and pitch. Conventionally, the position of the aircraft is represented by a symbol 1, called the aircraft icon, that is centred on a scale 2 graduated in degrees. This scale is referred to as the pitch scale. The symbol 1 is represented by a V, the wings of which include a horizontal bar. In FIG. 1, the scale 2 is represented by two series of symmetrical symbols 3 that are spaced apart by 5 degrees. The symbols 3 take the form of a square bracket. The ADI parameters also include a horizon bar 4;        The air speed scale 5. This is a vertical scale located to the left of the attitude scale. It represents the speed of the aircraft and is generally graduated in knots. In FIG. 1, the air speed is 155 knots;        The altitude scale 6. This is a vertical scale located to the right of the attitude scale. It is generally graduated in feet. In FIG. 1, the altitude of the aircraft is 1000 feet. This scale 6 is symmetrical with the speed scale 5 with respect to the ADI scale 2;        The speed vector 7. This is generally represented by a circle including two symmetrical horizontal segments and one vertical segment. It represents the angular direction formed by the speed of the aircraft with the angular position of the aircraft.        
There are two main flight modes, referred to as the heading-up mode and the track-up mode. In the case of an approach in heading-up mode, the pitch scale is centred on the aircraft icon. In the case of an approach in track-up mode, the pitch scale is centred on the speed vector.
The representation described above is perfectly suited to both modes as long as the aircraft icon and the speed vector remain relatively close to one another. Graphically speaking, this means that these two symbols remain within the pitch scale.
However, regardless of the flight mode, one of the two vectors will be isolated from the pitch scale because of a substantial drift of the aircraft due to a severe crosswind. In heading-up mode, the speed vector is isolated, due to a substantial drift, from any attitude reference. In the case of a go-around in track-up mode, the aircraft icon is isolated, due to a strong crosswind, in the flight view, without an accurate attitude reference, whereas this flight phase requires a determined attitude setpoint.
In order to solve this problem, multiple solutions have been proposed. The first solution consists in displaying, in addition to the speed vector, a reminder symbol positioned in the pitch scale that is representative of the pitch angle of the speed vector. This symbol consists, for example, of two symmetrical horizontal segments surmounted by a vertical segment. However, this symbol is not shown in a conformal position.
A second solution, when the symbolic representation is presented as superposed over a three-dimensional perspective view of the overflown terrain, consists in shifting the synthetic image, which is shown in a conformal position under normal conditions, so that the pitch scale remains centred on the viewing screen. The main drawback of these two solutions is that they display a symbolic representation or a representation of the exterior in a non-conformal position.